Multi-well filtration is a commonly used technique in chemistry and biochemistry for simultaneous filtering of suspensions contained in sample wells of a sample plate. Typically, the suspensions in the wells are first processed, for example, by eluting by means of a solvent or by using a reagent to precipitate one of the materials, and then the suspensions are filtered simultaneously to separate the desired material.
Conventional multi-well filtration assemblies typically comprise a filtration plate, or a separation plate, having a plurality of wells for receiving a liquid sample, and a collection plate having a plurality of wells for collecting filtrate. The filtration plate and the collection plate are disposed in a stacked relationship such that individual collection wells are aligned with a single filtration well. A conventional multi-well filtration plate, such as a microtiter plate, has 96 or 384 wells arranged in a 2:3 rectangular matrix for performing multiple assays simultaneously. Each well typically contains a separation media, for example a filter membrane, for separating a component from the fluid that is introduced into the separation plate, and allowing a liquid portion of the fluid to filter into the collection plate.
Depending on the application, either underpressure or overpressure is typically used in a multi-well filtration assembly to force the liquid through the separation media. Typically, the filtration assembly comprises a housing having means for producing the differential pressure.
Document U.S. Pat. No. 6,338,802 discloses a microfiltration apparatus for processing a plurality of fluid samples. The apparatus comprises a first plate having a plurality of columns, each column containing at one end thereof a filter element and a fluid discharge conduit beneath the filter element, and a second plate spaced apart from the first plate by a cavity, the second plate having a plurality of collection wells aligned with the columns for receiving sample fluid from the discharge conduits. The second plate also comprises a plurality of vents extending through the second plate adjacent the collection wells. The apparatus also comprises a gas-permeable material positioned in the cavity between the first plate and the second plate wherein the gas-permeable material is effective to permit a vacuum drawn from beneath the second plate to extend, via the vents, to a region above the second plate and to the plurality of columns, thereby drawing fluid from the columns into the collection wells and to obstruct movement of aerosols across the top of the second plate, thereby discouraging cross-contamination between the wells.
Document U.S. Pat. No. 5,141,719 describes a plate assembly for performing filtration on a plurality of samples. The assembly comprises an upper plate having a plurality of apertures, a single sheet of porous material of sufficient dimensions to span the entire plurality of the apertures, a rigid single-piece drop guide plate with a plurality of tubes incorporated therein, and a lower plate having a plurality of wells aligned with the apertures and the tubes. The assembly also comprises means for drawing a vacuum through the upper plate, drop guide plate and lower plate in order to draw liquid from the apertures through the tubes into the wells.
A drawback of known multi-well filtration apparatuses is that when using negative or positive pressure in a filtration process, a single defective sample well can disturb filtration in all the other wells, due to pressure leakage through the defective well. Known apparatuses have also turned out unreliable, some of the samples being filtered only partly in the filtration process. Problems have also arisen from drops dripping from a filtration well into a collection well at the end of and after the filtration process. Moreover, known apparatuses are difficult and complex to automate, and expensive to manufacture.